Our work is related to the potential role of polymorphonuclear leukocyte (PMN), i.e., bacterial interaction in the pathogenesis of tissue injury in periodontal disease. We have found that lysosomal enzyme release occurs in vitro when human or rabbit PMNs are exposed to bacterial plaques which adhere onto the cervical surfaces of teeth. When isolated plaque bacteria are incubated with PMNs, enzyme release can also be monitored in certain instances. Gram-positive isolates which are capable of forming plaques or causing periodontal lesions in experimental animals provoke PMN enzyme release. On the other hand "non-pathogenic" gram-positive bacteria in plaque do not stimulate PMNs. Gram-negative "pathogens" likewise cause PMN reactions but it is too early to say whether "non-pathogens" affect PMNs. We are attempting to characterize and modify the mechanism(s) of PMN-bacterial interaction and to correlate these in vitro data with the clinical severity of gingival and periodontal disease.